Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults
Clay smears in normal faults can form seals for hydrocarbons and groundwater, and their prediction in the subsurface is an important problem in applied and basic geoscience. However, neither their complex 3-D structure, nor their processes of formation or destruction are well understood, and outcrop...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-05-01
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| Series: | Solid Earth |
| Online Access: | http://www.solid-earth.net/7/789/2016/se-7-789-2016.pdf |
| _version_ | 1811205765780733952 |
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| author | M. Kettermann S. Thronberens O. Juarez J. L. Urai M. Ziegler S. Asmus U. Krüger |
| author_facet | M. Kettermann S. Thronberens O. Juarez J. L. Urai M. Ziegler S. Asmus U. Krüger |
| author_sort | M. Kettermann |
| collection | DOAJ |
| description | Clay smears in normal faults can form seals for hydrocarbons and groundwater,
and their prediction in the subsurface is an important problem in applied and
basic geoscience. However, neither their complex 3-D structure, nor their
processes of formation or destruction are well understood, and outcrop
studies to date are mainly 2-D. We present a 3-D study of an excavated normal
fault with clay smear, together with both source layers, in unlithified sand
and clay of the Hambach open-cast lignite mine in Germany. The faults formed
at a depth of 150 m, and have shale gouge ratios between 0.1 and 0.3.
The fault zones are layered, with sheared sand, sheared clay and tectonically
mixed sand–clay gouge. The thickness of clay smears in two excavated fault zones
of 1.8 and 3.8 m<sup>2</sup> is approximately log-normal, with values between
5 mm and 5 cm, without holes. The 3-D thickness distribution is
heterogeneous. We show that clay smears are strongly affected by R and
R' shears, mostly at the footwall side. These shears can locally cross and
offset clay smears, forming holes in the clay smear, while thinning of the
clay smear by shearing in the fault core is less important. The thinnest parts of
the clay smears are often located close to source layer cut-offs. Locally, the
clay smear consists of overlapping patches of sheared clay, separated by
sheared sand. More commonly, it is one amalgamated zone of sheared sand and
clay. A microscopic study of fault-zone samples shows that grain-scale mixing
can lead to thickening of the low permeability smears, which may lead to
resealing of holes. |
| first_indexed | 2024-04-12T03:37:01Z |
| format | Article |
| id | doaj.art-b292c1b1fa91435f91ce1e29d4d43635 |
| institution | Directory Open Access Journal |
| issn | 1869-9510 1869-9529 |
| language | English |
| last_indexed | 2024-04-12T03:37:01Z |
| publishDate | 2016-05-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Solid Earth |
| spelling | doaj.art-b292c1b1fa91435f91ce1e29d4d436352022-12-22T03:49:25ZengCopernicus PublicationsSolid Earth1869-95101869-95292016-05-017378981510.5194/se-7-789-2016Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faultsM. Kettermann0S. Thronberens1O. Juarez2J. L. Urai3M. Ziegler4S. Asmus5U. Krüger6Structural Geology, Tectonics and Geomechanics Energy and Mineral Resources Group, RWTH Aachen University, Lochnerstraße 4–20, 52056 Aachen, GermanyStructural Geology, Tectonics and Geomechanics Energy and Mineral Resources Group, RWTH Aachen University, Lochnerstraße 4–20, 52056 Aachen, GermanyChair of Geotechnical Engineering, RWTH Aachen University, Aachen, GermanyStructural Geology, Tectonics and Geomechanics Energy and Mineral Resources Group, RWTH Aachen University, Lochnerstraße 4–20, 52056 Aachen, GermanyChair of Geotechnical Engineering, RWTH Aachen University, Aachen, GermanyRWE Power AG, Cologne, GermanyRWE Power AG, Cologne, GermanyClay smears in normal faults can form seals for hydrocarbons and groundwater, and their prediction in the subsurface is an important problem in applied and basic geoscience. However, neither their complex 3-D structure, nor their processes of formation or destruction are well understood, and outcrop studies to date are mainly 2-D. We present a 3-D study of an excavated normal fault with clay smear, together with both source layers, in unlithified sand and clay of the Hambach open-cast lignite mine in Germany. The faults formed at a depth of 150 m, and have shale gouge ratios between 0.1 and 0.3. The fault zones are layered, with sheared sand, sheared clay and tectonically mixed sand–clay gouge. The thickness of clay smears in two excavated fault zones of 1.8 and 3.8 m<sup>2</sup> is approximately log-normal, with values between 5 mm and 5 cm, without holes. The 3-D thickness distribution is heterogeneous. We show that clay smears are strongly affected by R and R' shears, mostly at the footwall side. These shears can locally cross and offset clay smears, forming holes in the clay smear, while thinning of the clay smear by shearing in the fault core is less important. The thinnest parts of the clay smears are often located close to source layer cut-offs. Locally, the clay smear consists of overlapping patches of sheared clay, separated by sheared sand. More commonly, it is one amalgamated zone of sheared sand and clay. A microscopic study of fault-zone samples shows that grain-scale mixing can lead to thickening of the low permeability smears, which may lead to resealing of holes.http://www.solid-earth.net/7/789/2016/se-7-789-2016.pdf |
| spellingShingle | M. Kettermann S. Thronberens O. Juarez J. L. Urai M. Ziegler S. Asmus U. Krüger Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults Solid Earth |
| title | Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults |
| title_full | Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults |
| title_fullStr | Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults |
| title_full_unstemmed | Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults |
| title_short | Mechanisms of clay smear formation in unconsolidated sediments – insights from 3-D observations of excavated normal faults |
| title_sort | mechanisms of clay smear formation in unconsolidated sediments ndash insights from 3 d observations of excavated normal faults |
| url | http://www.solid-earth.net/7/789/2016/se-7-789-2016.pdf |
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